There is currently no support for shared Libraries so it will be filespace
consuming if you want to build X11-clients with X11R6. Best you mix X11R6
Server with X11R5 and X11R4 clients. And only compile clients who need the
new facilities provided in the X11R6 Libraries against them.

Please note:
XFree86 Version 4.0 will be the last version which is known
to compile on Interactive Unix.
I've only tested this version on Interactive Unix, Version 4.1mu.